# std::ranges::iota_view<W, Bound>::iterator

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< cpp‎ | ranges‎ | iota view

struct /*iterator*/; | (1) | (exposition only*)
Helper alias templates |  |
template< class I > using /*iota-diff-t*/ = /* see below */; | (2) | (exposition only*)
Helper concepts |  |
template< class I > concept /*decrementable*/ = std::incrementable<I> && requires(I i) { { --i } -> std::same_as<I&>; { i-- } -> std::same_as<I>; }; | (3) | (exposition only*)
template< class I > concept /*advanceable*/ = /*decrementable*/<I> && std::totally_ordered<I> && requires(I i, const I j, const /*iota-diff-t*/<I> n) { { i += n } -> std::same_as<I&>; { i -= n } -> std::same_as<I&>; I(j + n); I(n + j); I(j - n); { j - j } -> std::convertible_to</*iota-diff-t*/<I>>; }; | (4) | (exposition only*)

1) ranges::iota_view<W, Bound>::`iterator` is the type of the iterators returned by `begin()` and `end()` of ranges::iota_view<W, Bound>.
2) Calculates the difference type for both iterator types and integer-like types.

- If `I` is not an integral type, or if it is an integral type and sizeof(std::iter_difference_t<I>) is greater than sizeof(I), then /*iota-diff-t*/<I> is std::iter_difference_t<I>.
- Otherwise, /*iota-diff-t*/<I> is a signed integer type of width greater than the width of `I` if such a type exists.
- Otherwise, `I` is one of the widest integral types, and /*iota-diff-t*/<I> is an unspecified signed-integer-like type of width not less than the width of `I`. It is unspecified whether /*iota-diff-t*/<I> models `weakly_incrementable` in this case.

3) Specifies that a type is `incrementable`, and pre- and post- operator-- for the type have common meaning.
4) Specifies that a type is both `decrementable` and `totally_ordered`, and operator+=, operator-=, operator+, and operator- among the type and its different type have common meaning.

/*iterator*/ models

- `random_access_iterator` if W models `advanceable` (4),
- `bidirectional_iterator` if W models `decrementable` (3),
- `forward_iterator` if W models `incrementable`, and
- `input_iterator` otherwise.

However, it only satisfies LegacyInputIterator if `W` models `incrementable`, and does not satisfy LegacyInputIterator otherwise.

### Semantic requirements

3) Type `I` models `decrementable` only if `I` satisfies `decrementable` and all concepts it subsumes are modeled, and given equal objects a and b of type `I`:

- If a and b are in the domain of both pre- and post- operator-- (i.e. they are decrementable), then the following are all true:
  - std::addressof(--a) == std::addressof(a),
  - bool(a-- == b),
  - bool(((void)a--, a) == --b),
  - bool(++(--a) == b).
- If a and b are in the domain of both pre- and post- operator++ (i.e. they are incrementable), then bool(--(++a) == b) is true.

4) Let `D` denote /*iota-diff-t*/<I>. Type `I` models `advanceable` only if `I` satisfies `advanceable` and all concepts it subsumes are modeled, and given

- objects a and b of type `I` and
- value n of type `D`,

such that b is reachable from a after n applications of ++a, all following conditions are satisfied:

- (a += n) is equal to b.
- std::addressof(a += n) is equal to std::addressof(a).
- I(a + n) is equal to (a += n).
- For any two positive values x and y of type `D`, if I(a + D(x + y)) is well-defined, then I(a + D(x + y)) is equal to I(I(a + x) + y).
- I(a + D(0)) is equal to a.
- If I(a + D(n - 1)) is well-defined, then I(a + n) is equal to [](I c) { return ++c; }(I(a + D(n - 1))).
- (b += -n) is equal to a.
- (b -= n) is equal to a.
- std::addressof(b -= n) is equal to std::addressof(b).
- I(b - n) is equal to (b -= n).
- D(b - a) is equal to n.
- D(a - b) is equal to D(-n).
- bool(a <= b) is true.

### Nested types

Type | Definition
`iterator_concept` | an iterator tag, see below
`iterator_category` (only present if `W` models `incrementable` and /*iota-diff-t*/<W> is an integral type) | std::input_iterator_tag
`value_type` | `W`
`difference_type` | /*iota-diff-t*/<W>

#### Determining the iterator concept

`iterator_concept` is defined as follows:

- If `W` models `advanceable`, `iterator_concept` denotes std::random_access_iterator_tag.
- Otherwise, if `W` models `decrementable`, `iterator_concept` denotes std::bidirectional_iterator_tag.
- Otherwise, if `W` models `incrementable`, `iterator_concept` denotes std::forward_iterator_tag.
- Otherwise, `iterator_concept` denotes std::input_iterator_tag.

### Data members

Member | Definition
`W` `value_` | the current value (exposition-only member object*)

### Member functions

## std::ranges::iota_view::iterator::iterator

/*iterator*/() requires std::default_initializable<W> = default; | (1) | (since C++20)
constexpr explicit /*iterator*/( W value ); | (2) | (since C++20)

1) Value initializes `value_`.
2) Initializes `value_` with value.

## std::ranges::iota_view::iterator::operator*

constexpr W operator*() const noexcept(std::is_nothrow_copy_constructible_v<W>); |  | (since C++20)

Returns `value_`.

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(6, 9).begin()};
        const int& r = *it; // binds with temporary
        assert(*it == 6 and r == 6);
        ++it;
        assert(*it == 7 and r == 6);
    }

## std::ranges::iota_view::iterator::operator++

constexpr /*iterator*/& operator++(); | (1) | (since C++20)
constexpr void operator++(int); | (2) | (since C++20)
constexpr /*iterator*/ operator++(int) requires std::incrementable<W>; | (3) | (since C++20)

1) Equivalent to ++`value_`﻿; return *this;.
2) Equivalent to ++`value_`﻿;.
3) Equivalent to auto tmp = *this; ++`value_`﻿; return tmp;.

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(8).begin()};
        assert(*it == 8);
        assert(*++it == 9);
        assert(*it++ == 9);
        assert(*it == 10);
    }

## std::ranges::iota_view::iterator::operator--

constexpr /*iterator*/& operator--() requires /*decrementable*/<W>; | (1) | (since C++20)
constexpr /*iterator*/operator--(int) requires /*decrementable*/<W>; | (2) | (since C++20)

1) Equivalent to --`value_`﻿; return *this;.
2) Equivalent to auto tmp = *this; --`value_`﻿; return tmp;.

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(8).begin()};
        assert(*it == 8);
        assert(*--it == 7);
        assert(*it-- == 7);
        assert(*it == 6);
    }

## std::ranges::iota_view::iterator::operator+=

constexpr /*iterator*/& operator+=( difference_type n ) requires /*advanceable*/<W>; |  | (since C++20)

Updates `value_` and returns *this:

- If `W` is an unsigned-integer-like type:
  - If n is non-negative, performs `value_``` += static_cast<W>(n).
  - Otherwise, performs `value_``` -= static_cast<W>(-n).
- Otherwise, performs `value_``` += n.

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(5).begin()};
        assert(*it == 5);
        assert(*(it += 3) == 8);
    }

## std::ranges::iota_view::iterator::operator-=

constexpr /*iterator*/& operator-=( difference_type n ) requires /*advanceable*/<W>; |  | (since C++20)

Updates `value_` and returns *this:

- If `W` is an unsigned-integer-like type:
  - If n is non-negative, performs `value_``` -= static_cast<W>(n).
  - Otherwise, performs `value_``` += static_cast<W>(-n).
- Otherwise, performs `value_``` -= n.

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(6).begin()};
        assert(*it == 6);
        assert(*(it -= -3) == 9);
    }

## std::ranges::iota_view::iterator::operator[]

constexpr W operator[]( difference_type n ) const requires /*advanceable*/<W>; |  | (since C++20)

Returns W(`value_``` + n).

### Example

Run this code

    #include <cassert>
    #include <ranges>

    int main()
    {
        auto it{std::views::iota(6).begin()};
        assert(*it == 6);
        assert(*(it + 3) == 9);
    }

### Non-member functions

## operator==, <, >, <=, >=, <=>(std::ranges::iota_view::iterator)

friend constexpr bool operator== ( const /*iterator*/& x, const /*iterator*/& y ) requires std::equality_comparable<W>; | (1) | (since C++20)
friend constexpr bool operator< ( const /*iterator*/& x, const /*iterator*/& y ) requires std::totally_ordered<W>; | (2) | (since C++20)
friend constexpr bool operator> ( const /*iterator*/& x, const /*iterator*/& y ) requires std::totally_ordered<W>; | (3) | (since C++20)
friend constexpr bool operator<= ( const /*iterator*/& x, const /*iterator*/& y ) requires std::totally_ordered<W>; | (4) | (since C++20)
friend constexpr bool operator>= ( const /*iterator*/& x, const /*iterator*/& y ) requires std::totally_ordered<W>; | (5) | (since C++20)
friend constexpr bool operator<=> ( const /*iterator*/& x, const /*iterator*/& y ) requires std::totally_ordered<W> && std::three_way_comparable<W>; | (6) | (since C++20)

1) Returns x.`value_``` == y.`value_`.
2) Returns x.`value_``` < y.`value_`.
3) Returns y < x.
4) Returns !(y < x).
5) Returns !(x < y).
6) Returns x.`value_``` <=> y.`value_`.

The `!=` operator is synthesized from `operator==`.

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when iterator is an associated class of the arguments.

## operator+(std::ranges::iota_view::iterator)

friend constexpr /*iterator*/ operator+ ( /*iterator*/ i, difference_type n ) requires /*advanceable*/<W>; | (1) | (since C++20)
friend constexpr /*iterator*/ operator+ ( difference_type n, /*iterator*/ i ) requires /*advanceable*/<W>; | (2) | (since C++20)

Equivalent to i += n; return i;.

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when iterator is an associated class of the arguments.

## operator-(std::ranges::iota_view::iterator)

friend constexpr /*iterator*/ operator- ( /*iterator*/ i, difference_type n ) requires /*advanceable*/<W>; | (1) | (since C++20)
friend constexpr difference_type operator- ( const /*iterator*/& x, const /*iterator*/& y ) requires /*advanceable*/<W>; | (2) | (since C++20)

1) Equivalent to i -= n; return i;.
2) Let `D` be `difference_type`:

- If `W` is an integer-like type:
  - If `W` is signed-integer-like, returns D(D(x.`value_`﻿) - D(y.`value_`﻿)).
  - Otherwise, returns y.`value_``` > x.`value_``` ? D(-D(y.`value_``` - x.`value_`﻿)) : D(x.`value_``` - y.`value_`﻿).
- Otherwise, returns x.`value_``` - y.`value_`.

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when iterator is an associated class of the arguments.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2259R1 | C++20 | member `iterator_category` is always defined | defined only if `W` satisfies `incrementable`
LWG 3580 | C++20 | bodies of operator+ and operator- rule out implicit move | made suitable for implicit move

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